Kraft recovery boilers are used in the pulp and paper industry to recover usable energy from byproducts of the pulp making process. Kraft recovery boilers are similar to conventional fossil-fuel fired boilers. Black liquor fuel is introduced into the furnace along with combustion air. Inside the furnace, residual water is evaporated from the black liquor, and the organic material from the black liquor is combusted. The inorganic portions of the black liquor are recovered as sodium/sulfur compounds.
Gases generated by the black liquor combustion rise out of the furnace and flow across convection heat transfer surfaces. The vertical enclosure walls of the furnace are formed from heat transfer surfaces made of interconnected water tubes. Typically, feedwater enters the recovery boiler at the bottom of a first pass economizer, in which the water is heated as it flows to a steam drum. Saturated water is routed from the steam drum through pipe downcomers to lower furnace enclosure wall and floor inlet headers and a boiler bank. Natural circulation flow in the tubes is induced and driven by heat input to the vertical water cooled enclosure walls of the furnace from the combustion process.
Decanting floors in kraft recovery boilers are known for collecting and directing molten smelt from the black liquor combustion process to discharge openings in the boiler walls. The water tubes forming the floor are cooled by the circulation of water and/or a water/steam mixture through the tubes.
The floors of many known decanting recovery furnaces are essentially flat across the entire surface. Flat floors are subject to minor humping of the tubes, causing domes which form in the upper surfaces of the floor tubes. Steam can become trapped in these humps or domes (steam blanketing) which can cause the tubes to overheat and fail. More particularly, steam blanketing is where steam bubbles are not effectively entrained in the water moving through the tubes. In the flat floor tubes adjacent the furnace sidewalls, the heat input to the tubes may be lower. Since flow through the tubes is by natural circulation, the low heat input to these tubes results in lower fluid velocities and poor circulation in the tubes, which in turn causes steam blanketing.
Currently, the only known method for correcting this defect in flat floor furnaces is to replace the entire tube floor with a new floor. The new floor is sloped to increase the tolerance for heat absorption at lower fluid velocities and to permit venting of the minor humps which would otherwise trap steam and lead to tube overheat and failure. However, replacing the entire furnace floor is both time consuming and expensive, and a cost-effective solution would be welcomed by industry.